The parallel-hybrid drive trains as known in the art are usually constructed with a combustion engine, an electric machine, and a clutch device between the combustion engine and the electric machine. The electric machine is connected in a rotationally fixed manner with a transmission input shaft of a transmission which is positioned on the output side in reference to the electric machine, and the transmission represents different gear ratios. Thus, the rotational speed of the electric machine essentially equals the transmission input rotational speed.
During pure electric drive operation of a hybrid vehicle with a parallel-hybrid drive train, and where the clutch device between the combustion engine and the electric machine is disengaged, and the rotational speed of the electric machine is larger than an idle rotational speed of the combustion engine, and the combustion engine operates in an idle mode, the clutch device needs to be engaged for the operating dependent requirements to exist for coupling the combustion engine to the drive train, thus producing a hybrid operating mode.
If there exists a respective requirement to engage the clutch device, the rotational speed of the combustion engine is increased to the transmission input rotational speed, which is identical to the rotational speed of the electric machine, prior to engagement of the clutch device, such that the clutch device becomes synchronized. Hereby, the rotational speed of the combustion engine is rotation speed controlled, or also accelerated via a torque control in the direction of the synchronous rotational speed of the clutch device, which is engaged with little load when the synchronous condition is reached. Thereafter, the combustion engine generates the necessary load through the control of the torque of the combustion engine.
If the procedure of coupling the combustion engine, for instance, takes place during an uphill drive, which requires a large torque when in this traction mode, the combustion engine provides directly after engagement of the clutch device, as a maximum torque, a torque value which is derived from the rotational speed-torque-characteristic of the combustion engine.
Disadvantageously, the provided torque of the combustion engine, after the engagement of the clutch device at the related low rotational speed or synchronous rotational speed, respectively, of the clutch device, is not sufficient to transfer the load increase which is expected by the driver. That means that the vehicle in these operating conditions of the drive train, through the combustion engine, can at that time only be modestly accelerated. At the same time, due to the low rotational speed level from the combustion engine and a simultaneous large torque transfer of the combustion engine, it feels as though the engine will shortly stall.
A similar, disadvantageous operating condition sequence is present if the driver, during coupling of the combustion engine to the drive train, activates an accelerator pedal of the vehicle and spontaneously, for instance depending on the situation, when a significantly larger desired torque is required. At the time, in which the drive train is in the hybrid drive mode due to the synchronous engagement of the clutch device, the requested torque through the combustion engine, which is required for the conversion into the required acceleration of the vehicle, cannot be provided as spontaneously as expected.
During an operation condition sequence in which, during a deceleration mode of the parallel-hybrid drive train, a brake torque is requested from the combustion engine, the previously described, active acceleration of the combustion engine to the level of the electric machine or the transmission input rotational speed of the drive train, respectively, leads to unwanted fuel consumption by the combustion engine.
Furthermore, the request to couple the combustion engine to the drive train during an operating mode, in which a drive resistance changes significantly, is viewed as critical. That results from the fact that a vehicle, in traction mode with its parallel-hybrid-drive train and the presence of a spontaneous uphill drive, is delayed to a large extent so that coupling of the combustion engine is technically no longer appropriate and that the clutch device needs again to be disengaged, since in such a case the available traction force on the drive side is too low and a downshift in the transmission area needs to be initiated. Such an operating mode sequence creates high operating noise and leads to low drive comfort, an unwanted high disturbance, and creates stress to the parts.